The superstar won't join his teammates right away because of La Liga's COVID-19 protocol, but the Argentine will train individually until he is cleared. The club has multiple friendlies before beginning its league season on Sept. 27 against Villarreal.
Messi was initially hoping to leave Barcelona on a free transfer, but it would have required a dispute in court after disagreements over his opt-out date.
"Now I am going to continue in the club because the president told me that the only way to leave was to pay the €700 million (£624m/$823m) clause, and that this is impossible," he said in an interview with Ruben Uria of Goal.com.
Barcelona had also told the forward he could sit out the season if he wanted to leave for free.
The 33-year-old had a clause in his contract that allowed him to opt out by June 10, but he felt it should have been delayed because of the coronavirus pandemic that altered the season schedule. La Liga sided with the team in the dispute.
Manchester City was reportedly confident in landing Messi if he had transferred, reuniting the forward with coach Pep Guardiola.
Though he had stayed away from Barcelona during the disagreement, he is now back in the fold and ready to help the team for the upcoming season.
Barca are coming off a disappointing season in which they finished second to Real Madrid for the La Liga title and suffered an 8-2 loss to Bayern Munich in the Champions League quarterfinals. The club is hoping to get back to its standard of winning trophies and keeping the six-time Ballon d'Or winner provides a significant boost.